Injection molding and injection stretch molding is well known particularly in relationship to polyethylene terephthalate (PET). However, there are limitations to the use of PET products. In particular PET molded containers used in a hot fill process generally undergo undue deformation and therefore generally PET molded containers are not generally used with materials that require hot fill.
The process of stretch blow molding PET bottles is well known and involves the steps of manufacturing the preform generally by way of injection molding; heating the preform; and then stretch blow molding the heated preform. The manufacture of the preform is generally done remote from the blow molding of the bottles since the preforms are considerably smaller than the bottles and thus cheaper to transport. Accordingly, the manufacture of the bottles is often performed at the bottling site.
Polypropylene is a material that is currently only being used in a limited range of products. In particular the current polypropylene containers are generally opaque and thus not suitable for a wide variety of products. In particular in the food industry the consumer wants to be able to see the product being sold and thus products that are sold in opaque containers are not desirable.
Since the current methods of injection stretch blow molding PET preforms are very focussed on the processing characteristics of the polyester substrate these methods cannot be used for polypropylene. Specifically the machines used in these prior art methods cannot process polypropylene for the following reasons:
(a) the rate of heating of polypropylene is significantly slower than PET; PA0 (b) the processing window for polypropylene is much shorter than PET; PA0 (c) the technology to engineer preforms is significantly different than PET because of the stretch ratio differentials as well as the properties of the material itself; PA0 (d) heating patterns and thermal conductivity of polypropylene are very different from those of PET; and PA0 (e) the polypropylene preforms are thicker and bulkier than PET preforms. PA0 (a) slugging of material in the base or neck; PA0 (b) blowouts; PA0 (c) off center gates; and PA0 (d) bands of material in the neck or panel areas.
Accordingly, if current rotary wheel PET processing equipment is used to process polypropylene generally there are the following problems:
Many of these problems are related to the high degree of sensitivity to temperature variations of polypropylene.
Until now most attempts at injection stretch blowing polypropylene have been unsuccessful because of the lack of the precision in coordinating the appropriate heating of the external and internal walls of the preforms. Current heating devices, typically lamps, have been developed for heating PET and are positioned on the outside of the preforms to heat the outside thereof. In addition, the attempts have been unsuccessful because the machines developed for PET cannot accommodate the short processing window and the conditions for inflation which must be within very strict tolerances, much stricter than those required for PET.
Therefore with the development of new polypropylene including clarified polymer resins, co-polymer resins and random co-polymer resins it is now possible to produce, given the right processing conditions, a high clarity container made from polypropylene. Further, this container could be further enhanced with oxygen barrier chemistry technology.
Accordingly, it would be advantageous to provide a stretch blow molding apparatus and process for the manufacturing of polypropylene containers.